Create a dmz on cisco851w

I would suggest you need zone-based firewall (ZBF) for this, assuming it's supported in software available for the 851W.  Without ZBF you can still create a separate VLAN for the server, but routing will treat it as any other interface.  The fact that you say "DMZ" implies to me that you want to restrict traffic in various ways. You can restrict traffic with access-lists, but the dynamic nature of traffic these days makes stateful firewalls much more appropriate.

The VLAN portion of what you need is pretty easy to do, but ZBF is not especially easy for an inexperienced user to configure.  You can take a look at http://www.cisco.com/en/US/partner/docs/ios/sec_data_plane/configuration/guide/sec_zone_polcy_firew.html for information.  At a basic level, you create policies (allow HTTP, block telnet, etc.) that get applied to zones, and link interfaces with those zones.  In your situation, you'd most likely have outside, inside and DMZ zones.  Without knowing specifics of your topology and policies, it's going to be very difficult for anyone on this site to tell you exactly what commands to configure.

If you don't have software that supports ZBF, my recommendation would be to front-end the router with an ASA 5505.

ok so. i do not have zbf. what are the steps to configure my vlan on port f0/0

Thanks